Magnetic recording, reproducing, and erasing apparatus



Dec. 5, 1950 H. T. FAUS 2,532,

MAGNETIC RECORDING, REPRODUCING, AND ERASING APPARATUS Filed Jan. 9,1945 2 Sheets-Sheet 1 Inventor":

Harold'l". Faus,

li rs Attarney.

Dec. 5, 1950 H. 'r. FAUS 2,532,803

MAGNETIC RECORDING, REPRODUCING, AND IERASING APPARATUS Filed Jan. 9,1945 2 sheets sheet 2 Fig.5.

1 BEPPODUC/NG 37 III=\ y BE ANPL/F/EE Aw 05750702 5/ A N Invent-offHaPoldTFaus,

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Patented Dec. 5, 1950 MAGNETIC RECORDING, REPRODUCING, AND ERASIN GAEPARATUS Harold T. Fans, Lynn, Mass, assignor to General ElectricCompany, a corporation of New York Application January 9, 1945, SerialNo. 572,034

19 Claims. It

My invention relates to magnetic recording and reproducing apparatus andto methods for making the same. More particularly the invention relatesto processes and apparatus for recording and reproducing desiredsignals, such as sound or the like, upon magnetizable record mediahaving extended surfaces, such as disks, cylinders, cones, or the like.

It is a general object of my invention to provide a new and improved,and particularly a practical and inexpensive, magnetic recording andreproducing apparatus.

It is a further object of my invention to provide a new and improvedhigh coercive force magnetic recording medium of the disk or cylindertype which is practical and inexpensive to manufacture.

It is another object Of my invention to provide a new and improvedmethod for making magnetic recording media of the above type.

It is a still further object of my invention to eliminate cross talk orinterference from adjacent sound tracks in magnetic recording andreproducing apparatus of the disk or sheet type.

Still another object of the invention is the provision of new andimproved recording and reproducing heads for magnetic recordingapparatus of the disk or cylinder type.

It is a still further object of my invention to provide new and improvedmeans for erasing or demagnetizing a magnetic record sheet.

It is still another object of the invention to provide a new andimproved magnetic recording needle or stylus.

My invention will be more fully understood and its various objects andadvantages further appreciated by referring now to the followingdetailed specification taken in conjunction with the accompanyingdrawings, in which Fig. 1 is a perspective view of a magnetic recordingand reproducing apparatus embodying my invention; Fig.

2 is a cross-sectional view of the movable arm assembly including therecording and reproducing heads; Fig. 3 is a bottom view of the armassembly; Fig. 4 is a cross-sectional view of the arm taken along theline i-4 of Fig. 2; Fig. 5 is a cross-sectional view of a recording orreproducing head and a portion of the record sheet; Fig. 6 is aschematic circuit diagram, partially in block form, of a completemagnetic recording and reproducing system embodying my invention; Fig.'7 is a graphical representation of certain of the magneticcharacteristics of the record medium; Fig. 8 is a side elevation of amagnetic erasing head embodying m invention; Fig. 8A is a partiallysectionalized top view of the erasing head of Fig. 8 showing particularlthe distribution of flux adjacent the erasing head as it effects therecording medium; Fig. 9 is a cross-sectional view of a combinedrecording and reproducing head; and Fig. 10 is a diagrammaticrepresentation of a musical instrument including my new and improvedmagnetic recording and reproducing apparatus.

Referring now to the drawings, I have shown at Fig. 1 a disk recordingapparatus comprising a rotatable turntable I carrying a record sheet ordisk 2 and a movable arm 3 rotatably and pivotally mounted upon a base 4and carrying at its end a reversible portion in opposite side of whichare mounted recording and reproducing heads 5 and 6, respectively, inposition alternatively to engage the record disk 2.

The recording medium 2 is shown in greater detail at Fig. 5 andcomprises a disk-shaped supporting sheet I of homogeneou magnetizablematerial, such as soft iron or the like, having a rela tively lowreluctance and coated on each side with a thin layer 3 of magnetizablematerial having a relatively high reluctance and high coercive force. Itwill of course be understood that it is not necessary that both surfacesof the supporting disk T be coated, but that, if desired, only onesurface need be coated.

The high coercive force surface layers 8 are of uniform thickness andcomprise a pulverized mixture of magnetizable metallic oxides dispersedin a pliable organic binder, such as a resinous, cellulose or otherplastic material. Any of the well known thcrmosetting or thermoplasticmaterials, such a Bakelite and Vinylite, are well suited for the binder.

I have found that the oxides of certain metals are particularl welladapted, by reason of their particular magnetic properties, fordispersion in the binder to form a recording coating. Preferably,several such oxides are mixed together and pressed into a sintered massbefore grinding into a fine powder. One suitable material of thischaracter is provided by mixing together finely powdered magnetite,ferric oxide, and cobaltic oxide in the proportions of 43.6% magnetite,30.1% ferric oxide, and 26.3% cobaltic oxide. The mixture is then moldedto the shape desired under pressure of from three to five tons persquare inch. After molding, the material is removed from the mold andheated in an atmosphere of nitrogen or air for two or three hours atabout 1000 C. and allowed to cool. After cooling, the material is groundto a fine powder in which the size of all the particles is appreciablyless than the desired thickness of the coating 8. I have obtainedexcellent results with a coating 8 of about .004 inch in thickness, sothat the powdered mixture should be ground to a fineness of .0025 inchor less, and preferably less than .0001 inch.

The powdered magnetic oxide mixture prepared as above is then mixed witha suitable resinous or other plastic binder, for example Vinylite,preferably in the proportion of about 100 to 200 grams of the powderedoxide to on pint of the fluid plastic. This Viscous mixture is thensprayed or brushed upon th magnetic record base I to approximately thethickness desired. I have found, however, that in order to preventpeeling off of the oxide layer, it is necessary that the surface of basedisc I be treated prior to coating to increase its adhesion for theplastic binder, such as by bonderizing or the like.

After applying the magnetic coating to the record base i, the coatedrecord is heated suificiently to drive out any solvents present in thebinder and then allowed to cool. When a thermoplastic binder, such asVinylite, is used, this pre-heating may take'place at about 130 C. forabout four hours. In the event that a thermosetting plastic is used, itmay be necessary to use a lower temperature and a longer time so thatthe plastic does not take a permanent set.

After the solvents have been driven oif, the record is placed in a moldunder pressure in order to shape the record surface as desired. Athermoplastic record must be heated while in the mold to soften thesurface and then allowed to cool and set under pressure. A thermosettingmaterial must be heated in the mold so that setting will occur underpressure. I have found that ordinarily a pressure of about 1000 poundsper square inch is necessary to effect proper molding operation.

Preferably, the record mold includes a matrix formed to provide in therecording surface a guiding path, such as a groove or the like, forguiding a recording and reproducing needle. Ordinarily, the guiding pathor sound track will be in the form of a continuous spiral having aplurality of closely adjacent and substantially parallel convolutions.It will be understood, however, that if desired the record may be formedwith a plurality of separate paths, such as separate concentric circlesor separate concentric spiral paths. .004 inch in thickness, the recordgroove may suitably be of semicircular cross section with a radius ofapproximately .0015 to .002 inch and a spacing between grooves ofapproximately .01 to .015 inch.

It will of course be understood by'those skilled in the art that, whileit is desirable to form a sound track upon the recordsurface itself, itis also possible to provide other means, such as a threaded shaft or thelike, for guiding the recording and reproducing head in traversing thesurface of an ungrooved record.

As described in my Patent 2,248,616, issued July 8, 1941, the sinteredmixture of magnetizable oxide described above has highly desirableproperties-for the purpose here intended. Such material has a coerciveforce between 700 and 1000 cersteds, a permeability of the order of 1 to6, and a residual induction of about 2000 lines per square centimeter.The mixture of oxide and binder has When the recording surface 8 is astill lower combined permeability. Moreover, th hysteresis loop of theoxide mixture differs from those of known metals and alloys in that itevidences a greater linearity over a wide range of magnetizing forcesboth upon the rising and the falling portions of the hysteresis curve.The oxide has an electrical resistance between 600,000 and 1,000,000ohms per cubic centimeter, and is thus practically an insulator. Thematerial is of a gray slate color and, while hard, it is considerablymore friable than known magnetic metals or alloys. It is also very lightin Weight as compared with other magnetic materials, having a specificgravity of approximately one-half that of ordinary steel.

The magnetic record base I is formed of a material having a higherpermeability and lower coercive force than the surface coating material.Preferably the base I has a permeability of about 1000 and at least inexcess of 500. Suitable ma terials for the base 7 are soft iron orsteel, such as cold rolled steel, or silicon steel. I have found thatsilicon steel is preferable by reason of its relatively high electricalresistance. It will of course be understood that high resistance in boththe base 1 and the surface coating 8 is desirable in order to minimizeeddy currents in both portions of the recording medium. The record base7 may suitably have a thickness of approximately .03 inch, although ofcourse bases of greater thickness may be used if desired.

At Fig. '7, I have shown a magnetic hysteresis loop illustrative ofcertain of the magnetic properties of my record coating 8. In thisfigure, H represents magnetizing force and 3 represents the density ofmagnetization of the material. The curve 010 represents the manner inwhich the magnetization increases from zero upon the application of asubstantially saturating magnetizing force, and the curve pab representsthe manner in which the magnetization is brought back to zero byremoving and then reversing the magnetizing force. The ordinate 0arepresents the residual induction or retentivity of the material,

While the abscissa 017 represents the coercive force.

As will be more fully described hereinafter, the record surface ismagnetized by a needle in the recording head 5 which traverses thesurface of the record along a spiral path defined by the groovedsurface. The lines of force from the needle, the magnetization of whichis modulated in accordance with a desired signal, pass through theretentive coating 8 and into the record base 1. Upon removal of themagnetizing needle from any predetermined spot on the record, the baseI, being of low coercive 'force, loses substantially all of itsmagnetism at the point beneath that spot, while the coating 3 being of'high coercive forceretains the impressed magnetization. From Fig. 7, itwillbe evident that it is possible to carry out this recording operationeither upon the portion lip of the curve or upon the portion ad,depending upon whether the record before the impression .of the signalmagnetization was in an unmagnetized state or was uniformly magnetizedin one direction. In either case, it is desirable to provide therecording head with a unidirectional magnetic bias, as by direct currentor the like. For recording upon the rising portion tip of the curve ofFig. 7, such unidirectional bias avoids the nonlinear portion of thecurve adjacent the zero point 0. The purpose of a unidirectional bias,when recording upon the 'portionad of the curve, will be more fullyexplained hereinafter.

-rection of its thickness.

If the record surface is uniformly magnetized to the saturation value pprior to recording, the magnetization will, upon removal of the uniformmagnetizing force, decrease to some value, such as c, on the portion abof the curve. Such reduction of the magnetization below the residualvalue a results from the high self-demagnetizing effect of the thinsheet 8 magnetized in the di- Because of this effect, it will beobserved that the suitability of the magnetic oxide for the recordsurface resides more in its high coercive force than in its retentivity.So long as the coercive force is appreciably greater than theself-demagnetizing effect, materials having retentivities varying overwide limits will be seen to be equally suitable for the record surface.Moreover, it is the self-demagnetizing effect of the record whichrenders it necessary to provide a unidirectional magnetic bias whenworking upon the portion ad of the hysteresis loop. Without suchunidirectional bias, the positive portions of the signal wave would haveno effect. By unidirectional bias a further demagnetizing elfect isimpressed upon the record, so that the signal magnetization varies abouta mean value somewhat lower than the value 0 shown at Fig. '7.

The recording and reproducing heads and 6 are of similar structure, andone such head is shown at Fig. 5 of the drawing. Preferably, therecording and reproducing heads are not identical in practice, and thedifferences in the preferred forms of the heads will be made clear asthe description proceeds. In each case, the head comprises a coil spool9 of non-magnetic insulating material having an axial bore therethroughlined with a thin tube 10 of suitable magnetizable material, such as aniron-nickel alloy containing a high percentage, for instance betweenabout 30 and 80 percent, of nickel and characterized by a relativelyhigh permeability and low coercive force. The end flange of the coilspool 9 adjacent the record surface is conically flared externally toprovide a seat for a conically flared end plate H of magnetizablematerial, apertured at its apex and held in place by bolts I2. Thefunction of the end shield I I in reproducing operation will bedescribed in greater detail hereinafter. In the recording head themagnetizable shield II is preferably omitted, but may be included ifvery thin and easily saturable.

The axial bore in the coil spool 9 is tapered toward a small diameteraperture at the apex of the conical end flange in order to accommodate atapered needle I3. The pointed tip of the needle projects through theend of the coil spool and through the aperture in the apex of the endshield I i into engagement with the bottom of the spiral groove in therecord surface. Where no end shield H is provided, as in a recordinghead, the tapered end of the needle seats upon the tapered end of thespool bore. Where a magnetic shield I! is used, as in a reproducinghead, it is necessary that the point of the needle be closely anduniformly spaced from the shield. Such spacing may, if desired, beobtained by accurate machining, with the needle seating upon theinternally tapered spool body. I have found however that, in practice,it is desirable to allow the needle to seat upon the end shield llitself, as shown at Fig. 5, with a thin uniform non-magnetic spacer,such as a coating of varnish or the like, between the needle and the endshield.

Where the sound track groove has a crosssectional radius of about .0015inch as previously 6 described, the radius of the tip of the needle I3is preferably of the order of .001 inch.

The needle I3 is formed of any suitable highly permeable magnetizablematerial and, preferably, of a material having a relatively highelectrical resistance and low coercive force. Such a material may, forexample, be an alloy of 6% silicon and the balance iron, or an alloy of9.6% silicon, 5.4% aluminum and the balance iron. In a preferred form ofmy invention the tip of the needle is chromium plated to increase itsresistance to wear and, after such plating, the tip of the needle isdipped in varnish to provide the small uniform permanent air gap betweenthe needle tip and the magnetic shield H of the reproducing head. Itwill of course be understood by those skilled in the art that the 'aboveillustrative composition of the needle I3 is not to be regarded aslimiting in respect to my invention, but that other materials which aremagnetically soft and mechanically hard will be found equally suitable.For example, surface hardness at the needle tip may be attained bynitriding the point of the needle.

The needle I3 is of such diameter that it is freely slidable but notexcessively loose within the tube l0, and seats at the bottom eitheragainst the tapered end of the axial bore in the spool 9 or against theend shield l l, as preferred. The head of Fig. 5 is preferably areproducing head, and the needle is shown seated against the end shieldii. At it upper end the needle is firmly held in place against its seatby a set screw l5. Preferably, a resilient disk or washer I4 isinterposed between the end of the set screw !5 and the upper end of theneedle !3. The set screw 55 extends into the upper end of the tube IEand is threadingly mounted in the base of a cup-shaped magnetizableframe 56 which encases the Sides and one end of the spool 9 and engagesat its lower end the outer periphery of the end shield H. Thus, in arecording or reproducing head provided with a magnetizable shield II,the coil spool 9 and needle [3 are almost completely enclosed in amagnetizable casing, only the point of the needle extending through theaperture in the end shield II.

The magnetizable frame I6 is formed of any suitable highly permeablemagnetizable material, such as Permalloy, Nicaloi or Mumetal, and isheld. in place on the spool 9 b a pair of bolts H at the upper end ofthe head. The coil spool 9 carries a winding I 8 surrounding the needlefor the greater portion of its length and having a pair of leads !9extending through a suitable aperture in the side of the cup-shapedframe !6. In a recording head where it is desired to provide aunidirectional magnetic bias, a direct current may be superposed uponthe signal modulated current applied to the winding !8 or, if desired, aseparate direct current winding may be mounted upon the spool 9. Such awinding is shown at [8a in connection with the recording head 5 showndiagrammatically at Fig. 6.

The recordin and reproducing heads 5 and 6 are mounted in opposite sidesof the end of the movable arm 3, as shown in detail at Figs. 2 and 3.The arm 3 comprises an elongated hollow insulating support 20 open atthe bottom and having axially mounted therein a rotatable shaft 2|. Theelongated support 20 is pivotally and rotatably mounted adjacent one endupon the base 4 by means of a T-shaped plate 22. The plate 22 isrotatably mounted upon a vertical spindle 23 in the base 4 and is 7"provided atopposite endsof it cross arm". with upstanding lu'gspivotallyconnected tothe sides of the support 211 by pointed pins 25thereby toprovide both for pivotal movement of the arm 3 aboutahorizontal axis and for rotation ab'out a vertical axis.

The rotatable shaft 2 carried by the support 26- is journalled intheouter end of the support and extends beyond the end to carrya reversibleholder in which the recording and reproducing heads 5 and 6 are mounted.The holder 26' is pinned to the shaft 2! to rotate therewith, and ispreferably formed of anysuitable molded plasticihsulating material. Theopposit-e end of the shaft 25' is journalledupon a pin 21 projectingfrom the end of a threaded shaft 28 The shaft 28 is;,inturn, journalledat its opposite end in the other end of the support 2 9 and projectsbeyond the end of the support 21] to accommodate a linurled adjustingknob 29-. Intermediate its ends, the threaded shaft28 carries acounterweight 3i] threadingly mounted upon the shaft 2-8 and heldagainst rotation, but not against longitudinal movement, by engagementwith a suitable stop" (not shown) on the support 2!).

The shaft 2! is provided with detent means so arranged that the shaftmay be positioned only intwo alternative positions spaced apart by 180".mounted on the T'-shaped plate 22 remote from the pivot pins 25 andarranged for engagement with oppositely disposed holes 32 in the shaft25. By this arrangement, the movable arm 3 may be'lowered for engagementof a needle with the record surface only when one or the other of therecording and reproducing heads 5 and 5 is in proper position. When thearm israi'sed about the pivot pins 25', the shaft H is disengaged fromthe detent pin 3!.

The head" holder 28 is cylindrically recessed at its upper and lowersides to receive the record'- ing and reproducing heads 5' and 5. Theheads are held in place by set screws 33 in the end of'theholder, andsmall holes 3' 3 connect the base of each recess with the other side of'the holder toprovide access to the set screws H5 at the topof each head.The heads 5 and 6 are in side-by-side relation in the holder 2% and thecylindrical recesses in the holder are tilted slightly with respect tothe axis of the shaft 21' so that each head will be in" an optimumposition when it' needle is in engagement with the record disk 2..

From the foregoing description, it will be evident that the recordingand reproducing heads Eiand 6 are alternatively positioned forengagement with the'surface of'the record disk'z, and that the heads maybe interchanged by raising the arm 3' about the pivot pins 25 todisengage the detent 3E, 32; then reversing the head holder 28 by 180rotation of the shaft 21, and finally lowering the arm 3 so that theshaft 2'! is again locked in position by the detent pin 31.

The rotatable shaft 21' is also used t'o provide a simple drum typecontroller arranged to make suitable electrical'ccnnection betweentherecording and reproducing heads and a suitableradio receiver and signalamplifier. For this purpose;-

the shaft. 21 is preferably formed. of an insulat ing material andprovided on. opposite. sides. with a plurality of pairs of conductingsegments 3 35'; stand 31'? shownin greater: detail at Fig. l. Thesegments 3-'3i" are held in position. on. the? shaftby set screws 38 andprovidednwith terminal" This means comprises a pin 35 adjustably 7b thehighl permeability supporting; disk 1:, sur

tibnary ccnta'ct's- 42 connectors: 33E extending into. the hollowinterior oil the shaft 2! for connection with the necessary leadwires;The: surface of thesupporting arm: 2a is' provided with a plurality ofpairs of LIE-shaped. spring contacts-55 4i and 5'2 fastened to the armfit by bolts 43 and arrangedto engage those conducting segment ti t-31'-which are momentarily positioned at the upper side of the shaftl l The:controller segments S E-31 on the shaft ii and the contacts fill- 52 areshown schem'ati c'ally at Fig; 6 in asuitable: circuit connection toaconventional radio receiving apparatus At 6', I haveshowndiagrammatically a radio receiivin-g apparatus comprising anaerial44 connected to supply radio frequency energy to a radio: frequencyamplifier and signal detector shown in block for-In at L25. The detectoroutput i connected to the conducting segments 34" on the shaft 2 t. Theconducting segments 35" on the shaft 21 are connected to the lead Wiresfrom the coil it of' the recording head 5. The lead wires from: the coilL3 of the reproducing-- head 6 are connected tothe shaft-segments 35 onthe shaft 2- 1. The complete apparatusincludesalso-asignai amplifier 4 Ewhich'may'be of the electron-discharge type ordinarily used in homeradio receiving" sets" and a signal reproducing apparatus, suchas aloudspeaker (i8; The-input of: the signal amplifie 4? is connected tothe stationary contacts 56 shown in engagement with the conductingsegments 36 Thesignal amplifier outputi'sc'onnected' tothe stationarycontacts ST and the loud speaker input is connectedto the sta In' theposition of the-shaft 2i shown inthe drawing, the-shaft segments 3?bridgethe-contacts 4"! and 42-.

It will now be evident from Fig 6" that, with the shaft 25-positioneclas showmaudio frequency oscillations from the reproducinghead 6 are impressed upon the input of the signal amplifier Q'lZandfrom.the output of the amplifier uponthe loud speaker 452 If; on the otherhand, itisdesired toutilize the magnetic recording apparatus forrecording purposes, the shaft 2! is rotated about ahorizontal axisuntilthe conducting segments 34 and 35-show-n in the drawing attlielower side ofth'e shaft 21- are brought into engagement withthestationary contacts iii-42. In this position, the loud speaker 48 isdisconnected, the output of'the detector thereceivihg'appara'tu thisconnected to the input of the signal amplifier i? and the coil !8of'therecording'head 5 is connected to the outputofthe's'ignalamplifier.

m describing theoperation of my improved magnetic recording apparatus,let it'first' be assumed that the" shaft- 2! of the movable arm sis initsrecording position, so" that the head 5 is in engagement with therecord surface. In this position; signaloscillation are impressed uponthe'coil $8 ofthe head 5 from the output of the signal amplifier 4'2ofFi'g. 6 and the magnetiza tion of the-needle it varies in frequencyand in tensity in accordance with the frequency and-in tensity oftheiinpressed signal. nation is impressed uponthe recording surface 8"of the magnetic disk 2 by reason ofthe fact tha t th'ezn'iagn'eticcircuit ofs the head includes. a portiono'f: tiiedisk.v Referringparticularly to Fig; 5, the: magnetic circuit may be. followed from thepoint of the; needle through the recording surface 3 the base of: theslot engaged by the needle; and their inta distributed: path throughSuch magneti-- rounding portions of the recording surface 8, and backthrough air to the magnetic frame l6 forming the casing for the head.The upper end of the frame [6 is magnetically linked to the needlethrough the set screw and the magnetizable spool liner [0. It will ofcourse be understood that, in the event that a varnish coated needle isused as described hereinbefore, the Varnish is very rapidly worn off thetip of the needle, so that no permanent air gap is interposed betweenthe tip of the needle and the record surface.

In the flux path described above, the magnetic flux is considerablyconcentrated at the tip of the needle, and this concentration ismaintained as the flux passes through the recording surface 3. Thisconstricting effect arises from the fact that the flux evidences verylittle tendency to spread out from the point of the needle after itpasses into the surface 8 because of the high reluctance of the coating8 relative to the reluctance of the record base I. The distribution ofthe flux in the record base 1 is unimportant, and the coercive force ofthis base is sufficiently low that no appreciable amount of magnetism isretained after passage of the needle. It will be evident therefore thatby utilizing a material of relatively high reluctance in the recordingsurface layer, I am able to restrict the path of the fiux passingthrough the surface layer and thereby to minimize interferenc betweenthe magnetization impressed upon the record surface in the adjacent andsubstantially parallel paths defined by the closely spaced grooves.Furthermore, the high coercive force of the surface layer 8 ensures thata permanent record of the instantaneous magnetization will be retainedbetween the bottom of the groove and the top of the magnetically softbase disk '1. After the flux enters the base 1, it spreads out to suchan extent that no significant magnetization is impressed upon the recordsurface by the returning flux.

The high reluctance of the surface layer 8 is desirable not only for itsconstricting effect upon the flux passing through the surface layer, butalso for its effect in reducing the noise resulting from surfaceroughness in the record. It will of course be appreciated that, in eventhe most carefully prepared surface, a certain amount of unevennessexists which produces an irregular variation of the slight air gapbetween the tip of the needle and the record surface. Such variation inthe reluctance of the magnetic path tends to produce noise anddistortion. However, the. reluctance of the sintered magnetic oxidemixture which I have described heretofore is relatively so large, evenin the very thin coating described, that air gap variation by surfaceroughness is reduced to a relatively minor effect. This noise levelreduction is appreciable only when the permeability of the recordingsurface 8 is less than about 7. The mixed magnetic oxide and binderdescribed hereinbefore has a permeability of about 2. With such lowpermeability the noise reduction effect is quite marked.

From the foregoing description of the recording operation, it will beevident that the magnetizable end shield l l performs no useful functionin recording. The shield II, when present in the recording head,provides a shunt path for the useful flux from the tip of the needle tothe head casing it. It is therefore preferable to omit the shield l l inthe recording head, so that the useful flux will not be shunted awayfrom the record 2. It is possible, however, to provide a shield I I sothin that it may easily be saturated by the unidirectional biasing fluxsupplied by the biasing winding Isa on the recording head. When sosaturated, the shield H has little undesired shunting effect upon theuseful flux.

If now the head holder 26 is reversed, it will be evident from Fig. 6that the amplifier and detector 45 and the recording head 5 are bothdisconnected, while the reproducing head 6 is connected through thesignal amplifier i? and the drum controller to the loud speaker 48. Inreproducing operation, the head 5 traverses the record disk 2 and. thesignal modulated magnetization of the surface layer 8 beneath the grooveengaged by the needle modulates the magnetization of the needle 13,thereby to induce signal frequency currents in the winding I8. Thesesignal frequency currents are amplified in the signal amplifier 4! andimpressed upon the loud speaker 48.

In reproducing operation, the magnetizable end plate H acts as amagnetic shunt to prevent cross talk from those portions of the soundtrack immediately adjacent the groove engaged by the needle. Withoutsuch a magnetizable end shield, magnetic flux from grooves beneath thehead and adjacent the working groove may enter the needle and induceundesired signal oscillations in the recording coil. The magnetizableend plate 1 I on the reproducing head shields the needle from suchundesired magnetization by providing a shunt path through the shield l lfor flux from grooves adjacent the working groove. Thus, with a magneticend shield or shunt extending laterally from the tip of the needle, thereproducing head is substantially entirely enclosed in a magnetizablecasing, so that no magnetic flux can link the coil i8 except thatentering through the small needle aperture at the center of the shield.

A magnetized record disk may be erased either by magnetizing the diskuniformly in the direction of its thickness, or by completelydemagnetizing the disk. Erasure by uniform magnetization in thedirection of the thickness may be very simply accomplished by traversingthe sur-- face of the disk with a small permanent magnet, although ofcourse if desired any other source of constant magnetization, such as anelectromagnet, may be used. Preferably, the erasing magnet issufiiciently broad to cover a number of adjacent grooves on the record,and the erasing operation is carried out merely by moving the magnetradially across the disk while the disk is rotating. It will beunderstood that, when erasure is accomplished by uniform magnetizationin one direction, recording is carried out upon the portion ab of thehysteresis loop shown at Fig. '7.

Erasing may also be carried out by exposing the record to an alternatingflux of gradually diminishing intensity. At Fig. 8, I have shown anerasing head arranged to effect demagnetization in this manner. The headcomprises a support or base 55 in which are mounted two similar, butoppositely disposed, bar magnets 5i and 52 in parallel spaced relationwith opposite pole faces in substantially the same plane. Between themagnets 5| and 52 is mounted a needle 53 having its tip extendingslightly beyond the plane of the pole faces of the magnets 5| and 52. In

operation, the erasing head of Fig. 8 is mounted upon the end of amovable arm, such as the arm 3 of Fig. 1, with the needle 53 riding inthe spiral sound, track. As shown in Fig. 8, and in the top view of theerasing head in Fig. 8A, the magnets 51 and 52 are alignedlongitudinally of the groove engaged by the needle :53 so that :themagnets traverse the groove in following relation. in operation, eachportion of the sound track engaged by the needle 53 magnetized uniformlyin one direction by the leading magnet as the head approaches thatportion and the magnetization is immediately reversed by the trailingmagnet as the head leaves the portion. As the record rotates 36%)", thesame operation again takes place, but with slightly less intensitybecause the needle 53 is now engaging the adjacent groove. Thus, as maybe clearly seen from the flux distribution patterns indicated in dottedlines in Figs. 8 and 8A, as the erasing head progresses in the spiralgroove from the outer to the inner periphery of the record, each portionof the record surface experiences a number of successive reversals ofits magnetization with a gradually decreasing intensity.

At Fig. 9 I have shown a combined magnetic recording and reproducinghead generally similar to that of Fig. 5, but in which the magnetic endshield H is easily removable for use of the head without the shield inrecording. In this embodiment of the invention, the casing "it covers aportion of the tapered end of the spool 9 and is open at the top forinsertion of the spool. A magnetic end cap l-fia is placed over thespool after assembly in the casing. A spring Ma is shown between the setscrew l and the top of the needle 13. The spool lines it may be integralwith the cap Mia. The coil lab includes recording, reproducing andbiasing windings. The magnetic shield I! is mounted upon an arm 55 ofnon-magnetic material carried by a rotatable shaft 56 journalled in afixed bracket 51. The shaft 56 is movable longitudinally in the bracketand is biased to the position shown by a spring 58 between the bracket51 and a knurled knob 59 on the shaft. To remove the shield H from thehead the head and attached bracket 5'! are raised from the record, theshaft 56 depressed to release the shield ll, and the arm 5-5 turnedthrough 180 degrees. When the shield H is removed from the head, thespring I la moves the needle l3 downward slightly to seat upon the spool8. Suitable detent means may be provided to determine alternativeoperative and inoperative positions of the shield l I.

With a combined head such as that of Fig. 9, the end portion 25 of theswivel arm 3 need not be reversible, and the rotatable shaft 2! will notbe available for controlling the Winding connections. In this case,however, the controller contacts may be mounted upon the shaft 56, as at60 in Fig. '9.

At Fig. 10, I have shown schematically an arrangement wherein mymagnetic reproducing apparatus may be utilized as a musical instrumentfor producing desired sounds under the control of an operator. At Fig.9, the record disk 2 is provided with a plurality of separate concentricsound tracks 6], each of which is magnetized to produce a separate noteof uniform frequency. Since, as previously pointed out, the grooves 6!may be spaced as closely as .015 inch, it is evident that all the notesof the musical scale may be recorded upon a single record. A pluralityof reproducing heads 62 are mounted upon a common support and sopositioned that one head cooperates with each groove 61. The

heads 56 are connected in parallel-circuit relation through manuallyoperable keying switches 63 to supply one or more of the recorded notesto a common amplifying and reproducing apparatus '64.

It will now be evident that my new and improved recording andreproducing apparatus is inexpensive and easy to manufacture, and issimple, though versatile and durable, in operation. Record disks orcylinders made by the method described herein are particularly wellsuited for home recording and reproducing apparatus, since the recordsmay be utilized over and over again by the simple expedient of erasingthe previous recording with a small permanent magnet. The records aredurable in that the high coercive force recording surface will not peelofi', and the records can stand considerableshock, such as dropping andthe like, without appreciable change in the fidelity of theirreproduction. Furthermore, the confinement of the magnetic field inrecording operation, effected by the use of a thin recording surfachaving high coercive force and high reluctance, permits the use ofrelatively slow turntable speeds in both recording and reproducing. Iprefer to use '78 revolutions per minute, the usual phonograph turntablespeed, for both recording and reproducing operation although speech hasbeen very satisfactor-11y recorded and reproduced on my apparatus at 33revolutions per minute. The magnetic end shield on the reproducing headhas been found very successful in eliminating objectionable cross talkand thereby permitting closer spacing of the adjacent grooves andsmaller record s zes for a desired recording time. The slow speedoperation oi record, of course, contributes also to diminution in thesize oi the record for a given recording time. The fact that separateheads are desirable for recording and reproducing facilitates thecontroller arrangement which I have shown in connection with the shaft2!, so that my apparatus is particularly easily adaptable foralternative connection either to the incoming stages of a radioreceiving set for recording purposes or to the signal amplifying stagesof the set for reproducing purposes. Obviously, further controllercontacts may be added if desired. One such additional contact may, forexample, be used to control the biasing winding lila on the recordinghead. Additional contacts may be provided to connect the detector isdirectly to the amplifier 47, thereby to out out the recording apparatusand provide for direct receiving operation of the radio apparatus.

While I have shown only certain preferred embodiments of my invention byway of illustration, many modifications will occur to those skilled inthe art and I therefore wish to have it understood that I intend in theappended claims to cover all such modifications as fall within the truespirit and scope of my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates, is:

1. A magnetic recording and reproducing head comprising a frame, a coilspool carried by said frame and having an end flange and an axial bore,said axial bore being tapered toward a small aperture at the center ofsaid flange, a magnetizable needle positioned in said bore and having atapered end terminating in a fine point, and means for maintaining thetapered end of said needle seated at said aperture with said pointprotruding therethrough comprising a set screw mounted in said frame andarranged to apply axial pressure to the other end of said needle.

2. A magnetic recording and reproducing head comprising a cup-shapedmagnetic frame, a coil supporting spool disposed within said frame andhaving an end flange at the open end of said frame, said spool having anaxial bore tapered toward a small aperture at the center of said flange,a magnetiaable liner of low coercivity positioned in said bore, amagnetizable needle of low coercivity extending through said liner andhaving a tapered end terminating in a fine point, and a set screwmounted in the base of said cup and extending into said liner to applyaxial pressure to the opposite end of said needle thereby to maintainthe tapered end of said needle firmly seated with said point protrudingthrough said aperture.

3. A magnetic recording head comprising a cup-shaped magnetic frame, acoil supporting spool disposed within said frame and having a flared endflange protruding from the open end of said frame, said spool having anaxial bore flared toward a small aperture at the center of said flangeto form a conical internal shoulder, a magnetizable liner of lowcoercivity positioned in said bore, a magnetizable needle of lowcoercivity extending through said liner and having a flared endterminating in a fine point, and a set screw mounted in the base of saidcup and extending into said liner to bear against the opposite end ofsaid needle thereby to maintain the flared end of said needle firmlyseated upon said shoulder with said point protruding through saidaperture.

4. A magnetic recording and reproducing head comprising a magnetizableframe, a coil supporting spool mounted in said frame and including aflared end flange and an axial bore, said axial bore being taperedtoward a small aperture at the center of said flange, a magnetizabieneedle of low coercivity extending through said bore and having atapered end terminating in a fine point, a set screw mounted in saidframe and projecting into said bore to apply pressure to the oppositeend of said needle thereby to maintain said tapered end of said needlefirmly seated at said aperture with said point protruding through saidaperture, and a magnetizable plate substantially covering the outersurface of said end flange, said plate being centrally apertured toaccommodate the point of said needle and uniformly spaced from saidpoint.

5. In a magnetic recording and reproducing head including a needlehaving one end arranged to traverse a magnetic record surface, a frameof magnetizable material, a substantially nonmagnetizable core mountedwithin said frame and comprising an axial bore, means for mounting saidneedle within said bore means for shielding said needle from straymagnetic efiects comprising said frame, said frame comprising amagnetizable shunt plate generally frusto-conical in shape coaxiallydisposed with respect to and extending laterally from said one end ofsaid needle and with said one end protruding coaxially therefrom.

6. In a magnetic recording and reproducing head including a needlearranged to traverse a magnetic record surface, a frame of magnetizablematerial, a substantially non-magnetizable core mounted within saidframe and comprising an axial bore, means for mounting said needlewithin said bore means for shielding said needle from stray magneticeffects comprising said frame, said frame comprising a centrallyapertured flared plate of magnetizable material extending lateral- 14 1yfrom the point of said needle with said point projecting through saidaperture.

7. In a magnetic reproducing head including a needle having one endadapted to traverse a magnetized record surface, a frame of magnetizablematerial, a core of non-magnetizable material mounted within said frameand comprising an axial bore, a magnetic field winding mounted on saidcore, means for magnetically coupling said winding with said needlecomprising means for mounting said needle within said bore, said framecomprising an apertured plate of magnetizable material mounted adjacentsaid end of said needle with said needle extending through and spacedfrom the walls of said aperture, and means for detecting changes in themagnetization of said needle comprising said winding.

8. A magnetic recording and reproducing apparatus comprising a magneticrecord sheet, a magnetizable needle mounted to traverse said sheet, aframe of magnetizable material, a core of substantially non-magnetizablematerial mounted within said frame and comprising an axial bore, amagnetic field winding mounted on said core, means for magneti allycoupling said winding with said needle comprising means for mountingsaid needle within said bore and means for shielding said needle fromstray magnetic effects comprising said frame, said frame comprising acentrally apertured flared plate of magnetizable material extendinglaterally from the record engaging end of said needle, and means formagnetically coupling the other end of said needle to said framecomprising means for supportingly clamping said other end of said needleto said frame.

9. A magnetic reproducing apparatus including a record medium having asurface magnetized along a plurality of adjacent paths, a pick-up headincluding a magnetizable needle having a tapered end, means fortraversing at least one of said paths with said needle, and means forpreventing magnetic interference between a selected path and adjacentpaths comprising an apertured plate of magnetizable material mountedupon said head transversely of said needle with said tapered end of saidneedle extending through said aperture, said plate being tapered in thesame direction as said needle.

10. A magnetic reproducing apparatus including a record medium having asurface magnetized in a spiral path with adjacent turns closely spaced,a pick-up head including a magnetizable needle having a tapered end,means for traversing said spiral path with said needle, means forshielding said needle against magnetic interference from adjacent turnsof said spiral path comprising a centrally apertured flared plate ofmagnetizable material mounted upon said head with said tape-red end ofsaid needle extending through said aperture, said apertured plate beingflared in the same direction as the needle taper, and means including anelectric conducting winding surrounding said needle for detectingchanges in the magnetization of said needle.

11. A magnetic reproducing apparatus including a record medium having asurface magnetized in a spiral path with adjacent turns of said pathclosely spaced, a pick-up head including a coil spool having an endflange adjacent said record surface and an axial bore, a magnctizableneedle extending through said bore and having a tapered end arranged toengage said record medium in said spiral path, means for shielding saidneedle against magnetic interference from adjacent conassasoe volutions:of "said path including 'a flared :plate of magnetizable material,flared in the same direction as the needle taper, overlying said endflange and having a central aperture to accommodate said tapered end ofsaid needle, and means for effecting relative movement of said head 'andsaid'record medium whereby said needle traverses said spiral path.

12. .A magnetic reproducing apparatus including :a movable record mediumhaving a surface magnetized in a spiral path with adjacent turns of saidpath closely spaced, 'a pick-up head including a coil spool having anaxial bore, a magnetizable needle extending through said bore and having'a tapered end protruding beyond the End of said spool, and .amagnet'mable casing substantially completely enclosing saidspool andhaving an aperture through which the point of said needle protrudes toengage said record medium, said casing comprising a tapered portionextending from said aperture and tapered in the same direction as thetaper of said needle.

13. A magnetic recording and reproducing head comprising a coil spoolhaving an axial bore therethrough, a magnetizable enclosing casingsubstantially completely surrounding said coil spool and having acentrally apertured flared end adjacent one end of said axial bore, anda magnetizable needle having a tapered end terminating in a fine point,said needle being mounted within said borean'd having said tapered endprojecting through said aperture and spaced from the'walls thereof.

.14. A magnetic recording and reproducing head comprising a magneticenclosing casing having a centrally apertured flared end, a magnetizableneedle positioned within said casing and having a tapered end coatedwith a non-magnetic material and seated upon the Walls of said aperture,said tapered end terminating in a fine point outside said casing.

15. A magnetic reproducing head comprising a coil spool having a taperedend flange and an axial bore therethrough, a cup-shaped magnetizableframe encasing said spool with said end flange protruding from the openend thereof, a tapered magnetic shunt encasing said end flange andengaging the periphery of said frame, said shunt being centrallyapertured adjacent the end of said axial bore, and a magnetizable needlemounted within said bore and having a tapered point protruding throughand uniformly spaced from the walls of said aperture.

16. A magnetic recording and reproducing head comprising a coilsupporting spool having an axial bore, a magnetizable needle disposedwithin said bore and projecting from one end thereof, and a removablemagnetic shield for the projecting end of said needle comprising anapertured plate of magnetizable material swingably mounted upon an axisin substantially parallel spaced relation with said bore for movementinto and out of cooperative relation with said needle.

17. A magnetic recording and reproducing head comprising a coilsupporting spool having an axial bore tapered to a small aperture in oneend of said spool to form a conical internal supporting shoulder, amagnetizable needle positioned within said bore and having a tapered endprojecting from said aperture, resilient means biasing said needletoward seating engagement with said shoulder, an apertured plate ofmagnetizable material swingably'mounted upon an :axis in .substantiallyparallel spaced relation with said bore for movement transversely ofsaid needle into and out of operative relation therewith, said platewhen in said operative relation lying against said one end of said spoolwith said needle projecting therethrough, and means for biasing said:plate into engagement with said spool thereby to raise said needle fromsaid shoulder in seating engagement with said plate.

18. In combination, a magnetized record sheet having a spiral signaltrack, means for erasing said sheet comprising a pair of magnetic polepieces of opposite polarity disposed in a plane closely adjacent saidsheet, and means for traversing said spiral track with said pole piecesin following relation, the efiective fields of said pole pieces beingbroader than the pitch of said spiral, whereby each portion of saidtrack is repeatedlysubjected to fields of reversing polarity anddecreasing intensity.

19. In combination, a record sheet magnetized in a spiral signal path,an erasing head movable with respect to said sheet and including a pairof magnetic pole pieces mounted in spaced relation in a plane closelyadjacent said sheet, guiding means for constraining said head totraverse said path with said magnets in following relation, the pitch ofsaid spiral path and size and magnetic strength of said pole piecesbeing so interrelated that a plurality of separate portions of said pathare simultaneously aiiected by the fields of said pole pieces, therebyperiodically to subject each portion of said path to said fields atdiminishing intensity.

' HAROLD T. FAUS.

REFERENCES CITED ihe following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 900,392 Kirkegaard Oct. 6, 19081,202,638 Adom Oct. 24, 1916 1,343,507 I-Ieino June 15, 1920 1,416,216Legge May 16, 1922 1,781,711 Sumner Nov. 18, 1930 1,822,932 LoughridgeSept. 15, 1931 1,827,051 Thomas Oct. 13, 1931 1,828,190 Kilioni Oct.'20, 1931 1,847,860 Best Mar. 1, 1932 1,858,553 Liguori May 17, 19321,904,408 Charlin Apr. 18, 1933 1,940,274 Severy Dec. 19, 1933 2,078,357Woodmansee Apr. 27, 1937 2,089,287 Molloy Aug. 10, 1937 2,096,805Hickman Oct. 26, 1937 2,105,016 Smith Jan. 11, 1938 2,144,844 HickmanSan. '24, 1939 2,210,770 Muller-Ernesti Aug. 6, 1940 2,213,246 HellerSept. 3, 1940 2,293,413 Stoner Aug. 18, 1942 2,307,783 Malm Jan. 12,1943 2,335,930 Freeland Dec. 7, 1943 2,361,752 Eilenberger Oct. 31, 19442,361,753 Eilenberger Oct. 31, 1944 OTHER REFERENCES Barrett and Tweed,Some Aspects of Magnetic Recording and Its Application to Broadcasting.Available in Publications.

